These are some unmade monster concepts (from film and Videogame.) that never saw the light of day... Thanks to (TVTROPES for the info)

 

Deutalios

Deutalios was supposed to star in Godzilla vs. Biollante, in which he was fought, killed, and eaten by Godzilla, proving the Big G is carnivorous. He was removed and replaced by Biollante's rose form.

Dogolas

Very little is known about Dogolas. He only appeared on the CD-ROM The Godzilla Movie Studio Tour, and it is totally unknown what movie he was going to be in.

Firelion

 

One of four original monsters that were created for the video game Godzilla Unleashed and decided upon by a fan poll. Firelion lost to Obsidius and wasn't used for the final game.

A creature of myth and legend, Firelion is believed to have been the guardian of the lost civilization of Mu. The ruins of this once great culture were discovered sunken off the coast of Japan. Artifacts recovered from the site depict the existence of a great lion-like dragon with a fiery mane. This artist’s rendering is based on the many carvings and glyphs found amongst the ruins. Recent earthquake activity has opened fissures near the site which archeologists are eager to explore. According to the ancient images, Firelion possessed the ability to appear and disappear like a flickering flame, and throw fire from his tail.

Giant Mosasaur

 

A scrapped kaiju from Tokyo S.O.S, the Giant Mosasaur was going to appear as a corpse that was washed up on shore alongside the rock turtle Kamoebas, both of whom would be mauled and killed by Godzilla. It has no known abilities, and has never been considered for another film.

 

Lightning Bug

One of four original monsters that were created for the video game Godzilla Unleashed and decided upon by a fan poll. Lightning Bug lost to Obsidius and wasn't used for the final game.

An experiment to weaponize alien crystal energy goes terribly wrong when a tiny firefly buzzes into the focused energy stream. The firefly survives just long enough to lay her eggs in the test facility's cooling ponds. One of the eggs mutates and produces this uber bug. The Lightning Bug has hover flight, which allows her to unfold her wings and hover short distances. Her Armor Wings also serve as a shield from physical and energy attacks.

 

Mogu

Mogu is a dragon monster that was to appear alongside King Ghidorah and Gigan in the original draft for Godzilla vs. Gigan. However due to the initial enormous budget for the film, cut costs were made and Mogu (along with Rodan and Varan) was scrapped when Godzilla was given Anguirus as a partner instead.

The Visitor

 

One of four original monsters that were created for the video game Godzilla Unleashed and decided upon by a fan poll. The Visitor lost to Obsidius and wasn't used for the final game.

In their bid to retake earth, the Vortaak aliens have conscripted a new secret weapon into their arsenal of monsters. The Visitor is a feral alien creature captured from its home planet by the Vortaak with the intent of turning it loose on mankind. The monster clamps down on opponents with its massive jaws and stabs them with its crab-like appendages. The Visitor can also employ a Brainspasm, which shoots a directional shockwave from its pulsating cranium.